This invention relates to a liquid impregnation medium comprising metal impregnates belonging to the Group VIII and platinum group metals. The microemulsion impregnant contemplated comprises metal-containing microemulsions which are suspended in a liquid medium. The liquid impregnant medium is useful to the deisgning and manufacturing of catalysts comprising a specified range of metal particle size.
The size of a metal particle on a support may be very important to developing a catalyst which meets the activity, stability and selectivity requirements of a particular catalytic process. For example, it has been noted that synthesis gas conversion may be improved by controlling the particle size of the active metal on a catalytic support. M. Boudart, J. Phys. Chem. v. 88, (11), 1984. A means for controlling the size of metal particles to be impregnated is through the employment of microemulsions. The term "microemulsions" has been used to describe a variety of multi-component systems. In the instant invention a microemulsion is defined as a thermodynamically stable solution of water, hydrocarbon and surfactant which has the property of transmitting light equally in all directions. The microemulstions contemplated by the present invention are systems in which the hydrocarbon forms a solvent containing water in a reverse micelle, i.e., water pools surrounded by surfactant molecules. Each reverse micelle contains one water pool which may be thought of as an aqueous nuclei or water core. The water cores are isolated from one another by the nonaqueous environment.
The prior art discloses that metal salts may be dissolved into the individual water pools of the microemulsion. For example, in U.S. Pat. No. 4,425,261 reverse micelle microemulsions comprising hexane, penthaethylene glycol dodecylether (PEGDE), water, and respectively platinum, palladium, and rhodium salts were prepared. The metal compounds employed were dissolved and encapsulated in the water core of the reverse micelle. In order to obtain an impregnant of uniform metal particles, the micelle solution was first neutralized with an alkali base such as sodium hydroxide and the metal in the water core of the micelle was subsequently reduced with a reducing agent such as hydrogen and/or hydrazine. The metal particles formed in this way were uniform in size and did not deviate more than +/-10% in diameter. Only after the metal particles were reduced and metal flakes were formed within the water core could the metals be deposited on an oxide support. In other words, the impregnant disclosed by patentee necessarily requires the active impregnate metal to be present in a reduced valence state prior to contact with any oxide support.
The above described liquid suspension is believed to be the closest prior art disclosing reverse micelle containing impregnant solutions. Although patentee's liquid suspension is functional, it is not readily adaptable to commercial catalyst manufacture. First, the preparation of patentee's liquid suspension requires multiple steps leading to high economic inefficiency. For example, the reuse of starting materials and reagents, a common manufacturing technique, may be inhibited. Also, the impregnation of reduced metal particles on an oxide support does not generally result in an acceptable catalyst because the requisite thermal stability cannot be achieved. The transfer of colloidal metallic particles onto a support surface lacks sufficient metal-support chemical interaction, a feature which is generally accepted as a requisite for good catalytic action. Desirable metal-support interaction is gained usually when the metallic state is achieved in the presence of the support during catalyst finishing. In addition, the colloidal metallic particles in the patentee's description may also agglomerate during impregnation or transfer onto the surface of a support, leading to broad metallic size distribution.
The impregnant medium contemplated in the present invention avoids these problems by maintaining the metal impregnate ions contained in the water core of the reverse micelle in a nonreduced state. By the term "nonreduced state" it is meant that the metal is not present in the impregnant solution in a zero valence state, but rather is present as a dissolved metal ion. In accordance with the process for catalyst manufacture employing the reverse micelle containing liquid impregnant medium of the present invention and in contradistinction to the prior art, the metal ions contained in the water cores of the impregnant solution are reduced directly on the support surface after impregnation of the micelle.
The object of this invention is therefore to produce an impregnant medium comprising impregnatable metal containing reverse micelles. Further, it is an object of the present invention to prepare a liquid impregnant medium comprising reverse micelles which will yield uniform metal crystallites on an oxide support when contacted therewith. It is also an object of the present invention to reduce the number of steps required to form a stable liquid suspension or impregnant medium comprising reverse micelles. Finally, it is also an object of the present invention to provide a simple method of preparing the reverse micelle solution employed as an impregnant medium as well as a method for impregnated uniform metal crystallites on a catalyst support material. Further objectives, including the manufacturing of catalyst compositions having specific metal particle sizes and their use in various conversion processes as well as modifications which are obvious to those skilled in the pertinent art, are also contemplated.